In wireless networks, multiple devices typically share the same transmission medium or channel. This leads to the problem of medium access: If two or more devices access the shared medium at the same time, their transmissions will interfere with each other, leading to collisions and reduced system performance. The role of medium access control (MAC) is to moderate the access to the shared medium by defining rules that enable the devices to communicate with each other in an orderly and efficient manner. MAC protocols play an important role in ensuring efficient and fair sharing of the scarce wireless bandwidth.
One can broadly categorize MAC protocols into two types: centralized and distributed schemes. In infrastructure-based mobile networks, e.g. in GSM and UMTS, the medium access is typically controlled in a centralized manner. Dedicated network entities, such as base stations, allocate time slots or codes to mobile devices thus avoiding collisions.
In infrastructure-less mobile networks, so called ad hoc networks, the MAC must be performed in a distributed manner. The mobile nodes have to minimize collisions without global knowledge about the network. Current MAC approaches for ad hoc networks are usually based on the IEEE 802.11 standard for wireless local area networks (WLANs) which employs a distributed coordination function (DCF) that is based on a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism. The DCF includes two phases: (a) carrier sensing at the transmitter and (b) exchange of coordination information. The latter is performed by transmitting a request-to-send (RTS) message by the transmitter and a clear-to-end (CTS) message by the receiver in response to the RTS message. Neighboring nodes that overhear one or both messages have to abstain from transmission for the time period as indicated by the messages. This ensures the omni-directional spatial reservation of the shared medium around the transmitting and the receiving node of a communicating node pair.
In ad hoc networks, mobile devices communicate with each other in a peer-to-peer fashion; they establish a self-organizing wireless network without the need for base stations or any other pre-existing network infrastructure. If two devices cannot establish a direct wireless link (because they are too far away from each other), devices in between will act as relays to forward the data from the source to the destination. In other words, mobile devices can simultaneously act as data sources, data sinks, and intermediate forwarding devices. The mobile devices are referred to as “nodes”.
Neighboring nodes are silenced irrespective of whether or not they disturb signal reception. Reserving the channel in the area around the transmitter-receiver node is particularly suboptimal when beamforming antennas are deployed.
Beamforming is a technique that can be performed by so-called “smart” antennas, which consist of a plurality of antenna elements. With these elements, the mobile device can electrically form “beams” into advantageous directions instead of radiating the transmission power equally, i.e. in an essentially omni-directional manner. Depending on their capabilities, such antenna systems are referred to as “switched beam” antennas or “adaptive” antennas or phased array antennas. The term “beamforming pattern” refers to the entire direction-dependent antenna gain characteristic. Due to the limited number of antenna elements and the complexity involved, not the entire beamforming pattern can be controlled. Instead, there is usually an intended main beam, i.e., a sector with increased antenna gain, and several more or less unintentional side beams. This direction-dependent adjustment of the antenna gain can be performed both for transmission and for reception.
The present MAC protocols for ad hoc networks are not suitable when using beamforming antennas. The reasons are twofold. First, beamforming antennas potentially provide advantages over omni-directional antennas that cannot be exploited by present MAC protocols. Second, the use of present MAC protocols leads to side effects when beamforming antennas are deployed, which degrade the effectiveness and performance of the protocol.
The major advantage of using beamforming antennas in ad hoc networks is the potential for increasing the spatial reuse of the scarce radio resources. This potential resides in the fact that the signal-to-noise ratio (SNR), which is critical for effective data transmission, can be increased by forming beams in advantageous directions, while suppressing interference from other directions. Thus, a higher number of concurrent transmissions in some given area can be expected as compared to the case when omni-directional antennas are used.
If only the payload data is transmitted and/or received directionally, while the neighborhood of the receiver and the transmitter is still blocked as it is done with the IEEE 802.11 DCF, the MAC protocol will not exploit this potential benefit of beamforming antennas.
If, however, the before mentioned RTS and CTS messages are sent directionally, severe side effects are likely to happen. In this case, there may be nodes that are located within the transmission range of a currently receiving node, but they may not have received the RTS nor the CTS packed. These “hidden” nodes may then start a transmission, which may cause interference at said receiving node resulting in packet losses. Such situations are referred to as “hidden terminal” problems.
In general one can say that there is a tradeoff between spatial reuse and collisions (of control and/or data packets). The higher the spatial reuse is, i.e., the closer simultaneous transmissions sharing the same physical channel are, the higher is the number of hidden terminals and the probability of collisions.
A second major problem with beamforming antennas is known as “deafness”. It is assumed that there is a first node and a second node. It is further assumed that the first node wants to set up a communication with the second node. If the beamforming pattern of the second node is such that it suppresses the incident power from the direction of the first node, it will be “deaf” with respect to the first node.
For avoiding collisions, some known solutions propose an explicit distribution of collision avoidance information. With some solutions, a rather extensive amount of information is maintained that allows nodes to decide whether or not is it safe to transmit in a certain direction. The necessary information is usually included in control packets.
Another known solution deals with the tradeoff between spatial reuse and hidden terminals by directional transmission of RTS packets and omni-directional transmission of CTS packets.
Other known solutions use omni-directional tones together with directional control packets.
Further, there are solutions comprising a contention resolution phase, where transmission time slots are claimed, and a subsequent data transmission phase. However, such approaches necessitate synchronized mobile terminals.
For dealing with deafness, a known solution proposes to transmit tones upon packed transmission. These tones shall indicate to neighboring nodes that a deafness situation may have (possibly) occurred.
In the final conclusion the above makes clear that for beamforming situations as well as for non-beamforming situations existing networks are too restrictive and, thus, do not fully utilize the scarce transmission medium or are very complicated and, therefore, buy the optimum utilization for a high prize.